Laser based intersatellite communications systems have become attractive over the last decade, as lasers provide reasonable high output power emission in the fundamental spatial mode, and single frequency operation. The high antenna gain achievable at optical frequencies makes it possible to reduce the antenna diameter of the communications terminals and allows transmission at high data rates.
The most interesting scenario today is the interorbit link (IOL), where a low-earth-orbiting satellite (LEO) has to transmit high-rate data--up to several 100 Mbit/s--to a satellite in geosynchroneous orbit (GEO) serving as a data relay to a ground station. The data rate of the GEO-to-LEO link is much lower, in the range about several Mbit/s. The link distance is up to 45000 km. In order to compete successfully with microwave and millimeter wave systems, the optical terminals on the LEO spacecraft (user) have to be low-mass (&lt;50 kg) and low-cost. Moreover, it is desirable to operate a communication system having a mass which correspond to the required link capacity, so that a reduction of the data rate goes along with an equivalent reduction in mass and cost of the user terminal.
Typically, a terminal for a laser intersatellite communications system comprises a single telescope of about 25 cm in diameter, mounted on an optical bench together with all the optical components and front-end electronics. The optical bench, together with the telescope, can be moved under the control of a coarse pointing mechanism. The beam aperture in such an optical package is about 6 microradian.
For a beam divergence below 10 microradian, however, the pointing requirements become extremely stringent calling for prohibitively complex hardware and software. In practice, that leads to a limitation of the telescope diameter to some 30 cm. Consequently, the dimensions and the mass of the terminal are determined by the optical bench, the mechanical structure, the pointing mechanisms, the electronics, rather than by the dimensions of the optical antenna (telescope) itself. In other words, it results in a large and heavy terminal even though the mass of the telescope is just a fraction of the total terminal mass. Thus, a reduction of the telescope diameter would not significantly reduce the overall volume and mass, since they are mainly determined by the dimensions of the optical bench and of the coarse pointing mechanism.
Also known is a large aperture optical terminal comprising one very large single telescope (about 12 m in diameter) with a segmented sunshield (Optical Space Communication, Proceedings of the Meeting, Paris, France, Apr. 24-26, 1989). In this terminal designed by Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena, Calif., the telescope does not operate in the diffraction limit but as a photon bucket. This terminal is very well suited for receive-only systems and is intended for use in ground stations or in low-earth-orbiting satellites. The terminal has very high volume and mass, which leads to heavy gimbals whereby pointing is made more difficult. In addition, such a terminal requires a counter-terminal having a large aperture and a wide field of view for data acquisition whereby the design of the whole optical system becomes very difficult. Finally, this known optical terminal has poor transmit performance.
Another known optical terminal is the optical multiple access communications system (OMA) proposed by British Aerospace Public Limited Company. This system comprises a GEO-platform provided with several optical heads including a small telescope. Each telescope is pointed individually and each optical head thus requires an individual tracking loop and an individual receiver, which consequently increases the overall system mass. The individual operation of the receivers limits the achievable data rate to 1 Mbits/second for 175 mW transmitted average power.